Bars will be banned and tourist numbers limited on Koh Tachai as part of measures to protect the Similan Marine National Park in Phangnga province.
Park chief Nat Kongkesorn said on Saturday that bar owners had been told to demolish all structures they had built on the island.
Daily tourist numbers on Koh Tachai will be limited to 200, while just 250 people per day will be allowed on Koh Similan, Mr Nat said.
Tour agents will have to spread their clients around other islands such as Koh Mieng, Koh Paed and Koh Bua.
A quota will be given to existing tourism firms as well as to travellers who make online bookings for national park shelters. In a bid to preserve marine resources, swimmers and divers will be banned from going into waters beyond restricted areas. They will not be allowed to destroy marine and coastal resources, Mr Nat added.
Feeding marine animals such as turtles and fish will not be allowed, while urinating and defecating in the sea will be prohibited.
Park authorities have also ordered boat drivers not to run their engines at high revolutions as spinning propellers disturb marine plants and aquatic animals.
Boat companies have also been ordered to carry their waste back to the mainland to dispose of it in a landfill site in Thai Muang district.
The changes came after the Natural Resources and Environment Ministry was told Koh Tachai was at risk of environmental degradation in the form of spoiled coral and mounting rubbish as a result of a surge in the number of tourists.
Mr Nat said 16 firms operating tourist boats around the park were called in to help devise both short-term and long-term policies in a bid to limit damage to the environment.
Anyawarin Sujindasatien, the owner of Sea Star Co, agreed with park authorities that the number of visitors needed to be controlled and that the deteriorating state of marine resources should be addressed.
However, the measures will affect many operators as each firm received more than 150 bookings a day. About 1,000 tourists visited Koh Similan and Koh Tachai daily, she said.
It would be difficult to comply with the new regulations and control the number of visitors, she added.
When it came to transparency in collecting park entrance fees, Mr Nat said every company must distribute tickets to each customer to prevent bribery and corruption.
An aerial image showing blackened coral under a clear, turquoise sea around Koh Tachai was shared on Facebook last week. Mr Nat said the blue coral reefs in shallow water were damaged due to bleaching many years ago. The damage was not caused by oil or waste water released from Koh Tachai, he said.
There is a waste water treatment plant on the island, about one kilometre from the coastline.
It will not affect the ecological system in the national park, he added.